FDSOI (Fully Depleted Silicon-On-Insulator) transistors are of interest because of the possibility in achieving small transistor gate lengths with low OFF-state current. The gate dielectric thickness and the relevant body dimensions determine the short channel characteristics of FDSOI transistors, which may comprise an ultra thin body with low or no channel doping.
SiON (Silicon Oxynitride) is a candidate for a gate dielectric in a FDSOI transistor. SiON is a ceramic material composed of the system SiOxNy. Examples of SiON when in an amorphous form are SiO2 (Silica) and Si3N4 (Silicon Nitride). SiON can be grown on silicon and used as a gate dielectric layer.
FDSOI transistors having a SiON polysilicon gate structure have not been widely used due to the difficulty in obtaining an appropriate threshold voltage for conventional logic operation. In practice, the substrate of an FDSOI transistor should have very low doping in order to achieve little variation in the threshold voltage while maintaining a high carrier mobility, key advantages of FDSOI transistors. The polysilicon gate should be highly doped in order to eliminate polysilicon depletion effects.
However, with a combination of high polysilicon doping with low body doping, an FDSOI transistor with a SiON Polysilicon gate structure has a very low threshold voltage. Furthermore, when the gate length scales to sub-100 nm technology, the threshold voltage may drop to a negative value (for the N+ polysilicon case). Previous techniques to increase the threshold voltage to an appropriate value involve either an increase in substrate doping, thereby compromising the benefit of using an FDSOI transistor, or a decrease in polysilicon doping, thereby increasing the polysilicon depletion effect, leading to an increase in the channel inversion thickness.